The present invention relates to wire drawing dies, and more particularly to dies formed of a cemented metal carbide supported, polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN) compact wherein a non-cylindrical interface is provided between the compact and support layers for improved physical properties.
A compact may be characterized generally as an integrally-bonded structure formed of a sintered, polycrystalline mass of abrasive particles, such as diamond or CBN. Although such compacts may be self-bonded without the aid of a boning matrix or second phase, it generally is preferred, as is discussed in U.S. Pat. Nos. 4,063,909 and 4,601,423, to employ a suitable bonding matrix which usually is a metal such as cobalt, iron, nickel, platinum, titanium, chromium, tantalum, copper, or an alloy or mixture thereof. The bonding matrix, which is provided at from about 5% to 35% by volume, additionally may contain a recrystallization or growth catalyst such as aluminum for CBN or cobalt for diamond.
For many applications, it is preferred that the compact is supported by its bonding to substrate material to form a laminate or supported compact arrangement. Typically, the substrate material is provided as a cemented metal carbide which comprises, for example, tungsten, titanium, or tantalum carbide particles, or a mixture thereof, which are bonded together with a binder of between about 6% to about 25% by weight of a metal such as cobalt, nickel, or iron, or a mixture or alloy thereof. As is shown, for example, in U.S. Pat. Nos. 3,381,428; 3,852,078; and 3,876,751, compacts and supported compacts have found acceptance in a variety of applications as parts or blanks for cutting and dressing tools, as drill bits, and as wear parts or surfaces.
The basic HP/HT method for manufacturing the polycrystalline compacts and supported compacts of the type herein involved entails the placing of an unsintered mass of abrasive, crystalline particles, such as diamond or CBN, or a mixture thereof, within a protectively shielded metal enclosure which is disposed within the reaction cell of a HT/HP apparatus of a type described further in U.S. Pat. Nos. 2,947,611; 2,941,241; 2,941,248; 3,609,818; 3,767,371; 4,289,503; 4,673,414; and 4,954,139. Additionally placed in the enclosure with the abrasive particles may be a metal catalyst if the sintering of diamond particles is contemplated, as well as a pre-formed mass of a cemented metal carbide for supporting the abrasive particles and to thereby form a supported compact therewith. The contents of the cell then are subjected to processing conditions selected as sufficient to effect intercrystalline bonding between adjacent grains of the abrasive particles and, optionally, the joining of the sintered particles to the cemented metal carbide support. Such processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least 1300.degree. C. and a pressure of at least 20 kbar.
Regarding the sintering of polycrystalline diamond compacts or supported compacts, the catalyst metal may be provided in a pre-consolidated form disposed adjacent the crystal particles. For example, the metal catalyst may be configured as an annulus into which is received a cylinder of abrasive crystal particles, or as a disc which is disposed above or below the crystalline mass. Alternatively, the metal catalyst, or solvent as it is also known, may be provided in a powdered form and intermixed with the abrasive crystalline particles, or as a cemented metal carbide or carbide molding powder which may be cold pressed in to shape and wherein the cementing agent is provided as a catalyst or solvent for diamond recrystallization or growth. Typically, the metal catalyst or solvent is selected from cobalt, iron, or nickel, or an alloy or mixture thereof, but other metals such as ruthenium, rhodium, palladium, chromium, manganese, tantalum, copper, and alloys and mixtures thereof also may be employed.
Under the specified HT/HP conditions, the metal catalyst, in whatever form provided, is caused to penetrate or "sweep" into the abrasive layer by means of either diffusion or capillary action, and is thereby made available as a catalyst or solvent for recrystallization or crystal intergrowth. The HT/HP conditions, which operate in the diamond stable thermodynamic region above the equilibrium between diamond and graphite phases, effect a compaction of the abrasive crystal particles which is characterized by intercrystalline diamond-to-diamond bonding wherein parts of each crystalline lattice are shared between adjacent crystal grains. Preferably, the diamond concentration in the compact or in the abrasive table of the supported compact is at least about 70% by volume. Methods for making diamond compacts and supported compacts are more fully described in U.S. Pat. Nos. 3,141,746; 3,745,623; 3,609,818; 3,850,591; 4,394,170; 4,403,015; 4,797,326; and 4,954,139.
Regarding the sintering of polycrystalline CBN compacts and supported compacts, such compacts and supported compacts are manufactured in general accordance with the methods suitable for diamond compacts. However, in the formation of CBN compacts via the previously described "sweep-through" method, the metal which is swept through the crystalline mass need not necessarily be a catalyst or solvent for CBN recrystallization. Accordingly, a polycrystalline mass of CBN may be joined to a cobalt-cemented tungsten carbide substrate by the sweep through of the cobalt from the substrate and into the interstices of the crystalline mass notwithstanding that cobalt is not a catalyst or solvent for the recrystallization of CBN. Rather, the interstitial cobalt functions as a binder between the polycrystalline CBN compact and the cemented tungsten carbide substrate.
As it was for diamond, the HT/HP sintering process for CBN is effected under conditions in which CBN is the thermodynamically stable phase. It is speculated that under these conditions, intercrystalline bonding between adjacent crystal grains also is effected. The CBN concentration in the compact or in the abrasive table of the supported compact is preferably at least about 50% by volume. Methods for making CBN compacts and supported compacts are more fully described in U.S. Pat. Nos. 2,947,617; 3,136,615; 3,233,988; 3,743,489; 3,745,623; 3,831,428; 3,918,219; 4,188,194; 4,289,503; 4,673,414; 4,797,326; and 4,954,139. Exemplary CBN compacts are disclosed in U.S. Pat. No. 3,767,371 to contain greater than about 70% by volume of CBN and less than about 30% by volume of a binder metal such as cobalt.
As is described in U.S. Pat. No. 4,334,928, yet another form of a polycrystalline compact, which form need not necessarily exhibit direct or intercrystalline bonding, involves a polycrystalline mass of diamond or CBN particles having a second phase of a metal or alloy, a ceramic, or a mixture thereof. The second material phase is seen to function as a bonding agent for the abrasive crystal particles. Polycrystalline diamond and polycrystalline CBN compacts containing a second phase of a cemented carbide are exemplary of these "conjoint" polycrystalline abrasive compacts. Such compacts may be considered to be "thermally-stable" as compared to metal-containing compacts as having service temperatures above about 700.degree. C. Compacts as those described in U.S. Pat. No. 4,334,928 to comprise 80 to 10% by volume of CBN and 20 to 90% by volume of a nitride binder such as titanium nitride also may be considered exemplary of a thermally-stable material.
Supported PCD and CBN compacts have garnered wide acceptance for use in cutting and dressing tools, drill bits, and in like applications wherein the hardness and wear properties of such compacts are exploited. In particular, such compacts have been incorporated into dies for drawing feedstocks of such metals as tungsten, copper, iron, molybdenum, and stainless steel into wires. Typically, these wire drawing dies are configured as a generally cylindrical, inner mass of a PCD or CBN compact surrounded by and bonded to an generally annular, outer mass of a metal carbide support. Provided to extend through the compact along the axial centerline thereof is a hole or other aperture into which the metal feedstock is drawn for its elongation into a wire product of a reduced diameter. Wire drawing dies of such general type and methods for manufacturing the same are described in U.S. Pat. Nos. 3,831,428; 4,016,736; 4,129,052; 4,144,739; 4,303,442; 4,370,149; 4,374,900; 4,534,934; 4,828,611; 4,872,333; and 5,033,334.
With respect to the fabrication of the wire drawing dies herein involved, although a variety of methods may be employed, HT/HP sintering processes as are described in U.S. Pat. Nos. 3,831,428 and 4,534,934 may be considered preferred. As with the fabrication of supported compacts in general, the preferred HT/HP processes entail the sweep of a catalytic or binder metal, such as cobalt, through a mass of CBN or PCD particles. For wire die forming processes, the particles are charged within a support of a surrounding metal carbide annulus. At the processing conditions heretofore specified, metal from the support and, optionally, from an axially disposed disc, is made to infiltrate radially and/or axially into the interstices of the crystalline mass. Within the particle mass, the infiltrated metal forms a separate binder phase and, at least with respect to PCD, effects significant intercrystalline bonding. The metal additionally joins the sintered compact to the support to form an integral structure. The wire drawing hole may be formed through the sintered compact as a finishing step by laser drilling or other machining techniques. Alternatively, the hole may be pre-formed by including a wire as axially disposed within the particle mass, which wire is removed after the sintering of the mass by dissolution in a suitable acid or other solvent or by machining techniques.
As to supported compacts in general, it is speculated, as is detailed in U.S. Pat. No. 4,797,326, that the bonding of the support to the polycrystalline abrasive mass involves a physical component in addition to a chemical component which develops at the bondline if the materials forming the respective layers are interactive. The physical component of bonding is seen to develop from the relatively lower coefficient of thermal expansion (CTE) of the polycrystalline abrasive layer as compared to the cemented metal support layer. That is, upon the cooling of the supported compact blank from the HT/HP processing conditions to ambient conditions, it has been observed that the support layer retains residual tensile stresses which, in turn, exert a radial compressive loading on the polycrystalline compact supported thereon. This loading maintains the polycrystalline compact generally in compression which thereby improves the fracture toughness, impact, and shear strength properties of the laminate. In a wire die configuration, the support annulus has been observed, generally, to beneficially exert both an radial and an axial compression against the central polycrystalline core. However, localized regions of residual tensile stress are known to be present in the throat or reduction zone of the wire die.
During drawing operations, however, there are known to be developed frictional normal forces as between the contacting surfaces of the die and the wire being drawn. Such forces develop stresses which have been observed to combine with the residual stresses from the HT/HP forming process to deleteriously affect the operational life and performance properties of the die. Failure has been seen to occur principally within the bore of the die, or at the external, i.e., axial, surfaces of the compact layer.
Moreover, in the commercial production of supported compacts in general, it is common for the product or blank which is recovered from the reaction cell of the HT/HP apparatus to be subjected to a variety or finishing operations which include cutting, such as by electrode discharge machining or with lasers, milling, and especially grinding to remove any adherent shield metal from the outer surfaces of the compact. Such operations additionally are employed to machine the compact into a cylindrical shape or the like which meets product specifications as to diamond or CBN abrasive table thickness and/or carbide support thickness. With respect to wire drawing dies in particular, prior to use, the die generally is brazed into a receiving ring or other support assembly. It will be appreciated, however, that during such finishing and brazing operations, the temperature of the blank, which previously had been exposed to a thermal cycle during its HT/HP processing and cooling to room temperature, can be elevated due to the thermal effects of the operations. During each of the thermal cyclings, the carbide support, owing to its relatively higher coefficient of thermal expansion (CTE), will have expanded to a greater extent than the abrasive compact supported thereon. Upon heating and cooling, the stresses generated are relieved principally through the deformation of the compact layer which may result in its stress cracking and delamination from its support.
Proposals have been made to improve the performance of supported compact wire drawing dies. In this regard, U.S. Pat. No. 4,374,900 suggests surrounding the circumference of the diamond compact with a cermet material which contains molybdenum as a predominant component. The cermet is stated to have a high degree of plastic deformation and a high rigidity at elevated temperatures. U.S. Pat. No. 5,033,334 discloses a wire drawing die wherein the outer surface of the compact is metallized and then brazed to the mating surface of the support. Such die is stated to have an improved bond strength as between the compact and support components.
Notwithstanding the prior proposals, additional improvements in wire drawing dies would be well-received by industry. Especially desired would be a die having reduced residual stresses and, correspondingly, an extended service life, a reduced susceptibility to failure, and improved machinability, performance, and wear properties. Thus, there has been and heretofore has remained a need for wire drawing dies having improved physical properties.